This invention relates generally to devices used to test the shear response of a material, and more particularly relates to a portable apparatus and method for testing the viscoelastic response of a material specimen to an applied shear force under either monotonic or dynamic loading conditions. The device of the present invention may apply a circumferential force against the specimen perpendicular to a longitudinal axis of the specimen and may also apply a compression force against an end of the specimen.
Over the years, the Federal Highway Administration (FHWA) has been encouraging a modeling technique known as the SuperPave Asphalt Mix Design Model (hereinafter referred to as xe2x80x9csuperpavexe2x80x9d) as a method of predicting the life expectancy of various paving mixes. Paving mixes are typically custom tailored to the unique requirements dictated by local traffic, climate, materials selection, and structural section at the pavement site. The superpave model is intended as a useful tool to help estimate the pavement""s future long term performance in terms of its resistance to permanent deformation (rutting), fatigue cracking and low temperature cracking.
The superpave modeling technique requires the input of mechanical properties associated with the particular asphalt mix to be modeled. In order to determine the required input properties of the asphalt mix, several tests are performed to determine the linear and non-linear elastic response, viscous behavior, and tertiary creep tendencies of the asphalt mix sample. These tests are characterized by the application of dynamic and monotonic loads or strains in shear and thereafter measuring the resulting strain or stress response. The resulting test data is then implemented in the superpave modeling technique to estimate the life expectancy of the sample.
In order to most effectively estimate the life expectancy of a sample using the superpave technique, the test data should be obtained at the field level. Hence, a portable testing apparatus is desirable to perform the required tests on the sample in the field. To further increase the efficiency of obtaining the required test data, the sample material should not require substantial specimen preparation.
Often times, the sample test data is obtained in a laboratory setting using cumbersome testing equipment known in art as a Superpave Shear Tester (hereinafter the xe2x80x9cSSTxe2x80x9d). The SST includes a fixture that directs a shear load to a cylindrical specimen parallel with the ends of the specimen contained within the fixture. Proper use of the SST requires that the ends of a cylindrical specimen be cut square relative to each other and then glued to metal platens in a precision gluing jig prior to installation in the fixture. The xe2x80x9cglued specimenxe2x80x9d approach of the SST requires additional time and experience to properly glue and align the specimen. Further, in order to keep the ends of the specimen parallel, precise bearings are required to guide the specimen face as the shear load is applied. The use of bearings creates the possibility of backlash and misalignment. Hence, a need exists for a simple, easy to use, portable device for testing the shear strength of a material in response to an applied force.
Various fixtures have been developed that direct a shear load to a specimen contained within a fixture. In this regard, reference is made to the disclosures of Iosipescu et al., U.S. Pat. No. 3,566,681, Jones, U.S. Pat. No. 5,245,876, Terry, U.S. Pat. No. 3,406,567, Hall et al. U.S. Pat. No. 4,445,387, Peres et al., U.S. Pat. No. 5,280,730, Thompson et al., U.S. Pat. No. 5,289,723, and Buzzard, U.S. Pat. No. 4,916,954. These disclosures generally describe fixtures for applying shear loads to a specimen, but do not describe a fixture suitable for applying a circumferential force against a specimen perpendicular to a longitudinal axis of the specimen.
Iosipescu in U.S. Pat. No. 3,566,681 describes a method and apparatus for shear testing of rocks and other building materials. A rectangular block specimen is described, wherein a v-groove is formed in a middle, top and bottom portion of the block and channels, aligned with the grooves, are formed in the front and back of the block. The fixture described by Iosipescu applies a shear stress proximate the center of the v-groove and channel. A cylindrical specimen held in the fixture described by Iosipescu would tend to rotate within the fixture as the shear force is applied. Further, the clamping of the rectangular specimen by the fixture does not provide for through-zero loading. Also, the fixture described by Iosipescu does not use flexures for maintaining the distance between the two clamping assemblies.
Jones in U.S. Pat. No. 5,245,875 describes a fixture whereby a shear stress is applied to specimen with rectangular beam geometry. Jones describes using the fixture to shear polymeric materials and does not describe an active split clamping system to provide through zero loading and to prevent the specimen from rotating within the fixture. Further, Jones does not describe a fixture that includes flexures for maintaining the distance between the two specimen attachments and there is no mention of measuring specimen strain.
McRae in U.S. Pat. No. 5,911,164 discloses a compaction and pavement design testing machine and method for testing flexible pavement materials. The device described by McRae provides a rotational or gyratory shear testing. The compacting device described by McRae is not simple and portable and further does not apply a force that is perpendicular to the longitudinal axis of the specimen.
Vilendrer in U.S. Pat. No. 5,712,431, describes a device for testing the shear response of a material in response to an applied force. The ""431 patent describes applying a shearing force to a cylindrical specimen along the longitudinal axis of the specimen. The specimen could potentially rotate within the fixture as the shear force is applied to the specimen. In contrast to the device described in the ""431 patent, the device of the present invention applies a circumferential force against the specimen perpendicular to a longitudinal axis of the specimen, which may be in combination with a compression force against the ends of the specimen.
Thus, there is a need for a device, used for testing a response of a material specimen to shear forces applied to the material specimen, that applies the shear force against the specimen perpendicular to a longitudinal axis of the specimen, that may also apply a compression force against an end of the specimen, and which inhibits twisting or rotation of the specimen as the shear force is applied. The present invention meets these and other needs that will become apparent from a review of the description of the present invention.
The purpose of the present invention is to provide a portable field shear tester for determining shear stress test data corresponding to various asphalt mixes that can subsequently be used in modeling methods to estimate the future pavement life. The present invention provides a shear tester, wherein the shear strengths of the specimen can be tested with minimal preparation at the field site and with great speed. Testing of the asphalt material is performed by obtaining a cylindrical sample and placing it into a shear fixture that can be subjected to monotonic or dynamic forces, including a circumferential force against the specimen perpendicular to a longitudinal axis of the specimen. The specimen may also be subjected to an additional compression force against an end of the specimen.
The shear-testing device of the present invention includes a base, first and second clamps, and corresponding flexures. The first clamp is attached to the base and clamps about a perimeter of the specimen in proximity to a distal end of the specimen. The second clamp is attached to the first clamp via flexures and clamps about the perimeter of the specimen in proximity to a proximal end of the specimen. The clamps fasten about the perimeter of the specimen with enough force to inhibit rotation of the specimen within the clamps. Without limitation, in the preferred embodiment opposite halves of each clamp are forced together with hydraulics. The flexures have a proximal and distal end, wherein the proximal end of the flexures is attached to the second clamp at the proximal end of specimen, and the distal end of the flexures is attached to the first clamp at the distal end of the specimen. A downward force to the second clamp causes the proximal end of the flexures and the second clamp to move downward while the first clamp remains stationary, thereby applying a circumferential force against the specimen perpendicular to a longitudinal axis of the specimen.
An additional actuator may be coupled to the first clamp to thereby apply a compression force against an end of the specimen. Front and back plates are affixed to the first and second clamps, whereby the plates engage the respective ends of the specimen. The additional actuator applies a force against the plate engaged with the distal end of the specimen. In order to measure the relative displacement between the first and second clamps, when the circumferential or compression force is applied to the specimen, linear displacement transducers may be utilized to measure the same.
The shear fixture is coupled to a monitoring and control system that includes a microprocessor-based servo-controller, which controls, via closed loop feedback, the amplitude and frequency of the applied load or displacement to the shear fixture. A microprocessor-based temperature controller is also used to control the environmental control system temperature.
Those skilled in the art will appreciate that, although the preferred method of testing is to apply a load and thereafter measure the resulting displacement, an alternative test method would be to displace the sample a predetermined distance and then measure the load required to displace the sample the predetermined distance. To provide more control over the material properties, the specimen temperature is held constant by enclosing the shear fixture in an environmental control chamber.
Without limitation, in the preferred embodiment shear tests can be performed to stress levels of 1200 KPA (with 700 KPA supply pressure) and strains to 12% at frequencies from 0 to 10 Hz. The device can perform various tests including a frequency sweep, simple shear and repeated shear to obtain relevant data corresponding to each test, the data of which is required in the superpave modeling technique. The applied load, specimen dimensions, and measured displacement are then analyzed to determine the material stress/strain of the specimen associated with the required test data properties. These properties, along with the controlled temperature, may then be used in the superpave modeling technique to thereby estimate the material""s long-term performance.
Further, both dynamic (sinusoidal or pulsed) or static loading can be applied to the shear fixture. A servo pneumatic shear load actuator having a shaft coupled to the shear fixture is used to create the applied load. A servo valve, mounted near the shear load actuator, ports high-pressure supply air (typically 80-175 psi) to either side of an actuator piston. The resulting imbalance of air pressure creates the desired load or force in the desired direction. To energize the servo shear actuator separately from the air supply, an on/off solenoid valve is provided. Those skilled in the art will appreciate that although a servo pneumatic actuator is used to generate the loading, other known load generators could be used including, for example without limitation, a servo hydraulic, electrodynamic or electromechanical actuator.
An environmental control chamber surrounding the shear fixture may be a box type configuration with a door for sealably enclosing the fixture and specimen. The chamber preferably has both hot and cold capability and features an electric heater assembly and liquid CO2/N2 injector for cooling. Those skilled in the art will appreciate that while a chamber enclosure is used, other heating and cooling means including heating and cooling the material retaining clamps directly could be used. The chamber may include a temperature sensor for temperature readout and control. A signal corresponding to the resulting temperature is transmitted to the microprocessor-based temperature controller for monitoring and control purposes and can be used to ensure that the test is being run at a specific temperature. In this manner, a shear tester is provided that may be used for testing asphalt specimens, for example, at the field site for the purpose of generating test data that can be used in the superpave modeling technique.
These and other advantages of the present invention will become readily apparent to those skilled in the art from a review of the following detailed description of the preferred embodiment especially when considered in conjunction with the claims and accompanying drawings in which like numerals in the several views refer to corresponding parts.